Rains and snow in late November of last year formed a pond in one end of Racetrack Playa that froze over. When it thawed, winds pushed huge ice sheets around the pond, sometimes moving rocks along with them, some as far as 700 feet. When the pond dried up in February, there were tracks left in the mud.

It turns out, it doesn't take much ice to do the trick, only about the thickness of a windowpane.